Question: In history class, the girl to boy ratio is $3$ to $5$. If there are $12$ girls, how many boys are there?
Solution: A ratio of $3$ girls to $5$ boys means that there is a group of $3$ girls for every group of $5$ boys. If there are $12$ girls, then there are $4$ groups of $3$ girls each. Because we know the ratio is $3:5$ , there must be $4$ groups of $5$ boys each. There is a total of $20$ boys in history class.